Rare-Earth  Oxyorthosilicate Scintillator Crystals and Method of Making Rare-Earth Oxyorthosilicate Scintillator Crystals

ABSTRACT

A method of making LSO scintillators with high light yield and short decay times is disclosed. In one arrangement, the method includes codoping LSO with cerium and another dopant from the IIA or IIB group of the periodic table of elements. The doping levels are chosen to tune the decay time of scintillation pulse within a broader range (between about ˜30 ns up to about ˜50 ns) than reported in the literature, with improved light yield and uniformity. In another arrangement, relative concentrations of dopants are chosen to achieve the desired light yield and decay time while ensuring crystal growth stability.

RELATED APPLICATION

This is a continuation application of the U.S. patent application Ser.No. 13/276,446, filed on Oct. 19, 2011, which is a continuationapplication of the U.S. patent application Ser. No. 12/967,442, filed onDec. 14, 2010. The U.S. patent application Ser. Nos. 13/276,446 and12/967,442 are incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to scintillator materials used for detectingionizing radiation in nuclear imaging applications particularly PET(Position Emission Tomography), TOF PET (Time of Flight PositronEmission Tomography) and/or DOI PET (Depth of Interaction PositronEmission Tomography) imaging. This invention relates particularly tocontrol of decay time, rise time and scintillation light yield of rareearth oxyorthosilicates. Specific arrangements also relate to control ofdecay time, rise time and scintillation light yield of rare earthoxyorthosilicates.

BACKGROUND

Lutetium oxyorthosilicate (LSO), or Lu₂SiO₅ activated with cerium(Ce³⁺), is a well-known crystal scintillator material and widely usedfor medical imaging, such as gamma-ray detection in positron emissiontomography (PET) as well as other applications. Due at least partly toits relatively high light yield and short decay time, LSO is consideredto be one of the most suitable materials for molecular imagingapplications specifically for time-of-flight PET (TOF PET).

LSO scintillators are typically made of single-crystal LSO grown from amelt using for example, the Czochralski process. For scintillatorapplications, it is often desirable to be able to grow largesingle-crystals of LSO with specific optical performance parameters. Thesize and quality of the grown crystals can be significantly affected bythe growth stability.

While LSO scintillators in general have been well developed, efforts areon going to develop LSO scintillators with improved properties forspecific applications.

SUMMARY OF THE DISCLOSURE

The present disclosure relates generally to LSO scintillators with highlight yield and short decay times, and method of making suchscintillators. In one arrangement, the method includes codoping LSO withcerium and another dopant from the IIA or IIB group of the periodictable of elements. The doping levels are chosen to tune the decay timeof scintillation pulse within a broader range (between about ˜30 ns upto about ˜50 ns) than reported in the literature, with improved lightyield and uniformity, while ensuring crystal growth stability.

A further aspect of the present disclosure relates to LSO scintillatorswith improved optical characteristics, including decay times tunablebetween about ˜30 ns and about ˜50 ns. In one configuration, the LSOscintillators include LSO doped with Ce and another dopant from the IIAor IIB group of the periodic table of elements, where the concentrationsof Ce and the other dopant in the melt each range from about 0.001% toabout 10% (atomic percentage used throughout the present disclosureunless otherwise specified; nomenclature used in the references topapers and patents remain consistent with the original text used bydifferent authors).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an LSO crystal grown with high concentrations of Ce and Ca(Ce 0.1% and Ca 0.05%), where crystal growth instability problemsoccurred during the growth process.

FIG. 2 shows an LSO crystal grown with appropriate adjustment of Ce andCa concentrations (Ce 0.025% and Ca 0.1%). The conditions of the crystalgrowth was the same as those used for growing the crystal shown in FIG.1, except for the Ce and Ca concentrations The crystal growth wasstable.

DETAILED DESCRIPTION I. Overview

This disclosure relates to scintillator materials used for detectingionizing radiation in nuclear imaging applications particularly PET(Position Emission Tomography), TOF PET (Time of Flight PositronEmission Tomography) and/or DOI PET (Depth of Interaction PositronEmission Tomography) imaging, where PET imaging is inclusive ofdedicated PET, and hybrid PET imaging modalities such as: PET/CT(Positron Emission Tomography with Computed Tomography capabilities),PET/MR (Positron Emission Tomography with Magnetic Resonancecapabilities) and PET/SPECT (Positron Emission Tomography with SinglePhoton Emission Computed Tomography capabilities). This disclosurerelates particularly to control of decay time, rise time andscintillation light yield of rare earth oxyorthosilicates.

Lutetium oxyorthosilicate (LSO) or Lu₂SiO₅, invented by Charles L.Melcher and described in U.S. Pat. No. 4,958,080, is a well-knowncrystal scintillator material that is widely used for gamma-raydetection in PET as well as other applications.

LSO traditionally used in nuclear imaging scintillator devices has thegeneral chemical formula Ce_(2x)Lu_(2(1-x))SiO₅ and is typically dopedwith Ce in the range 0.001≦x≦0.1 (i.e. 0.1% to 10%) in the melt, whileother impurities are limited to low levels. Cerium ions play a role asextrinsic luminescence centers, or activators, in the material,producing ultraviolet or blue light under excitation with high energyionizing radiation (for example, gamma, x-ray, beta, alpha radiations).LSO has a density 7.4 g/cm³, relatively high light yield (up to about75% of NaI:Tl), and fast decay time (41 ns). LSO is widely used in thefield of medical imaging. The original composition of LSO crystalcontains cerium as the only intentional dopant. Crystals are grown usingthe Czochralski technique with a well defined growth atmosphereconsisting primarily of an inert gas with a few percent of oxygen. LSOis currently considered to be the most suitable material for molecularimaging applications specifically for TOF PET.

TOF PET has been of interest for medical imaging for some time. Ishii etal. (Ishii K, Watanuki S, Orihara H, Itoh M, Matsuzawa T. Improvement oftime resolution in a TOF PET system with the use of BaF₂ crystals. Nucl.Instr. Meth. In Phys. Research 1986; A-253:128-134) reported one of thefirst TOF PET systems built based on BaF₂ scintillator material. BaF₂ ofa density 4.89 g/cc, effective Z of 52.2 and decay time of 600 ps wasthe best scintillator available for TOF PET application at that time.However, BaF₂ has multiple limitations for TOF PET: It has light outputof only 5% of NaI:Tl, secondary slow component 620 ns of decay time andpoor stopping power for 511 keV. In contrast, cerium activated rareearth oxyorthosilicates with inherent physical properties of higherdensity and effective Z along with much higher light yield and shortdecay times are more suitable for TOF PET.

Researchers skilled in the art of PET have known that the convolution ofseveral parameters such as: decay time, rise time and light output arecritical for TOF PET scintillators. Shao et al. (Shao Y., “A new timingmodel for calculating the intrinsic timing resolution of a scintillatordetector”, Phys. Mec. Biol. 52 (2007) 1103-1117) provides a comparisonof these parameters of several scintillators of interest for TOF PET.GSO:Ce, a rare earth oxyorthosilicate with multi-decay time componentshas been used in medical imaging.

Sumiya K. et al., GSO single crystal and scintillator for PET, (U.S.Patent Application Publication No. US2003/0159643 A1) investigated anddemonstrated the effect of cerium concentration on the scintillationproperties. GSO codoped with Ce shows two components in thescintillation time profile of cerium emission. The fast component decayswithin the range of 30-60 ns and the slow component between 400-600 ns.Sumiya was able to reduce the relative contribution of slow and fastcomponents in scintillation time profile (reducing output ratio) andvary the fast component of the decay time from 60 ns to 35 ns byincreasing concentration of Ce from 0.5% to 1.5%, respectively. However,the change in the Ce concentration reduces light output and degradesenergy resolution. Sumiya shows that this effect can be associated withthe presence of the Ce⁴⁺ ions causing yellow coloration. Sumiya alsoattempted to further modify the scintillation properties of GSO byeliminating the coloration by codoping with one of the elements from Mg,Ta and Zr. GSO:Ce single crystal codoped with one of these impuritiesare colorless and the transmission of its emission wavelength is notreduced even if Ce concentration is about 1.5%. His teaching furtherdemonstrates that output ratio of slow component is reduced to above ½time and the decay time is faster than that for GSO:Ce by a factor of ⅓.

Inherent characteristics of LSO (density of 7.4 g/cc, effective Z of65.5, single component decay time of about 41 ns, and light output of75% of NaI:Tl) makes it a highly suitable PET scintillator. However,applying the teachings of Sumiya with respect to varying Ceconcentration in LSO for the purpose of changing decay time does notsubstantially produce the desired result. This demonstrates that theteaching of Sumiya cannot be successfully applied to all rare earthoxyorthosilicates. Sumiya's teachings are limited to GSO:Ce. Zavartsevat al. (Zavartsev Y. D., Koutovoi S. A., Zagumennyi A., “Czochralski,growth and characterization of large Ce³⁺:Lu₂SiO₅ single crystalscodoped with Mg²⁺ or Ca²⁺ or Tb³⁺ for scintillators”, Journal of CrystalGrowth 275 (2005) e2167-e2171) showed that highly doped LSO:Ce(0.15-0.22 at %) provides 75% of light output of NaI:Tl and decay timeof about 41 ns. Low-doped crystals have lower light output and decaytime of 44-45 ns. However, for highly doped LSO:Ce crystals two maintypes of imperfections are observed: constitutional supercooling finescattering and yellow color, attributed to the lattice defects near Ce⁴⁺ions. This observation is consistent with the teaching of Sumiya.Zavartsev also teaches that codoping with trivalent ion Tb³⁺ results ina non-radiative energy transfer from Ce³⁺ to Tb³⁺ ions that reducesdecay time to 34 ns but also reduces light output by about 50%; and thatthe crystals are colorless for Ce concentration of 0.2% and Tb 1%.Higher concentrations of Ce and Tb result in a light green color.Zavartsev further teaches that addition of divalent ions Ca²⁺ or Mg²⁺into the melt containing tetravalent Ce⁴⁺ or Si⁴⁺ or Zr⁴⁺ ions suppressthe tendency of spiral growth and cracking of the crystals that wasbased on gallium garnets growth technology. Zavartsev further teachesthat the concentration of oxygen vacancies is important for thescintillation mechanism because it involves the recombination process ofmany electron-hole pairs during gamma-excitation, as photoluminescenceefficiency of Ce³⁺ emission in oxides crystals including LSO:Ce. Thequantity of oxygen vacancies is controlled with the acceptor dopant, forexample Ca²⁺ and Mg²⁺ ions. Calcium or magnesium dopants fix theconcentration of oxygen vacancies in crystals for example YAG:Ca,Ce.Zavartsev observed that during cutting, LSO:Ce:Mg crystals have atendency to crack in contrast to, LSO:Ce:Ca crystals. Zavartsev shows(at Table 1 of the Zavartsev reference) that LSO:Ce:Mg and LSO:Ce:Ca areboth colorless and have essentially the same light output as LSO:Ce. ForCe concentration of 0.25% and Mg 0.02% LSO:Ce:Mg exhibits 39 ns decaytime. For Ce concentration of 0.22% and Ca 0.02% LSO:Ce:Ca exhibits 41ns decay time. LSO:Ce:Ca exhibits fairly strong afterglow attributed tothermal activated de-trapping of charge carriers from deep trapsfollowed by electron-hole recombination at Ce³⁺. This finding is incontrast to certain data presented by Ferrand et al. (Ferrand et. al,Dense high-speed scintillator material of low afterglow, U.S. PatentApplication Publication No. US2010/0065778 A1), where it is disclosedthat introduction of a divalent alkaline earth metal ions M substitutingfor a trivalent rare earth ion, or trivalent metal ions M′ substitutesfor a tetravalent silicon atom, creates a positive charge deficit thatlimits the trapping of electrons responsible for the afterglow. Ferranddiscloses that the addition of a divalent alkaline earth metal M and/ortrivalent metal M′ to LYSO type composition substantially reduceafterglow. In particular, M may be Ca, Mg, or Sr (in divalent cationform). In particular, M′ may be Al, Ga, or In (in trivalent cationform), with element M substituting for Y or Lu, and element M′substituting for Si. The introduction of a divalent alkaline earth metalions M and/or trivalent metal ions M′ was for the specific purpose ofreducing the afterglow. (See also, PCT Patent Publication WO 2006/018586A1.)

Chai et al (Chai B. H. T. Ji Y. Lutetium yttrium orthosilicate singlecrystal scintillator detector, U.S. Pat. No. 6,624,420 B1) teaches thatCe doped LSO has several serious problems. They include: (1) traceamount of naturally occurring long-lived radioactive isotope ¹⁷⁶Lu, (2)LSO crystals have very deep traps defects evidenced by very longphosphorescence after exposure to a UV light source, (3) crystal growthrequires very high melting temperatures 2200° C., which is detrimentalto insulation and iridium crucibles used for growing the crystals, and(4) high cost of lutetium oxide raw material. Chai states that thematerial purity 99.99 is not sufficient to guaranty the consistent lightyield. It is, in his teaching, highly desirable to replace Lu₂O₃ as themain ingredient in new scintillator crystals, namely LYSO. The LYSOinventors addressed these problems by (1) substituting Lu with Y toreduce ¹⁷⁶Lu content in the scintillator, and (2) lowering the crystalgrowth temperature by 100° C. For the trap problem, Chai et al.concluded that the crystallization process that has been found to be apurification process implies that the top portion of the crystal boulehave the least impurity content and would have the best light yieldperformance. Rapid reduction of light yield occurs when crystal growthis progressing and a greater fraction of the melt is converted to thecrystal. They go on to state that this is consistent with all publishedspeculations that impurities are the primary cause of creating deeptraps that give long phosphorescence and reduces scintillation lightyield. They concluded that the impurities are coming from Lu₂O₃ startingmaterial. To reduce phosphorescence it is necessary to reduce thelutetium content by yttrium substitution. To address detriment toinsulation and Iridium crucibles they reduce temperature by 100° C. bysubstituting substantial amount of Lu with Y. To adjust the problem withhigh cost of lutetium oxide they substituted lutetium with yttrium downto as low as 70% substitution (degradation of the light yield occurs formore than 70% of substitution).

All of the disclosures and publications citied above are directed tosolving problems associated with phosphorescence (afterglow), ortransmittance where the inventors either use codoping with divalent ortrivalent ions or substituting of lutetium by yttrium in the case ofLSO. In the case of GSO the inventors additionally made the attempt tochange scintillator decay time by changing the Ce content. However, thischange compromised the transmittance and light yield of thescintillator. To address the transmittance problem, GSO was codoped withmetal ions.

Zagumennyi et al. (Zaguemnnyi A. I., Zavartsev Y. D., Studenekin P. A.,Scintillating substance and scintillating wave-guide element, U.S. Pat.No. 6,278,832 B1) made an attempt to change multiple properties ofLu-based scintillator materials: increase in the light output ofluminescence, decrease of the time of luminescence of Ce³⁺, increase ofthe reproducibility of properties of grown single crystals, decrease ofthe cost of the source melting stock for growing crystals scintillatorsdue to the large amounts of Lu₂O₃ needed, and prevent cracking duringmanufacturing scintillation elements. Some of his teaching is explainedbased on the example of LSO:Ce scintillator. Zagumennyi teaches thatdifferent displacement of oxygen ions after the substitution ofCe³⁺→Lu₁, Lu₂ in coordination polyhedron LuO₇ and LuO₆ determinepractically different scintillation characteristics of the material. Thelight output, the position of the luminescence maximum and the constantof the time of scintillations decay (time of luminescence) depend on thenumber of Ce³⁺, which substituted ions Lu₁ and/or Lu₂. Thus in gammaexcitation, both centers of luminescence are excited and luminescencesimultaneously, and the time constant for scintillation decay willdepend on the duration of luminescence of both the first and secondcenters and on the relationship of the concentration of ions of Ce³⁺ incoordination polyhedrons LuO₇ and LuO₆. The center of luminescence Ce₁(polyhedron LuO₇) has a time of luminescence of 30-38 ns and theposition of the luminescence maximum 410-418 nm; the center ofluminescence Ce₂ (polyhedron LuO₆) has a time of luminescence of 50-60ns and the position of maximum luminescence of 450-520 nm. Zagumennyiteaches that the best technical result is observed in scintillatingcrystals containing ions Ce³⁺ only in polyhedrons LuO₇. The simultaneouspresence of Ce³⁺ ions in LuO₇ and LuO₆ decreases the light output by3-10 times, increasing the time of luminescence up to 40-50 ns andshifts the luminescence maximum into the area of less sensitivity ofphoto multiplier tubes. Moreover, he teaches that the crystalscontaining ions of Ce³⁺ advantageously in coordination polyhedrons LuO₇are produced from melts additionally doped with ions of the followingelements: Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W. By that, ions of Ti, Zr,Sn, Hf, Nb, Sb, Ta occupy in the crystal lattice the position withoctahedral coordination (polyhedron LuO₆) due to higher bond energies ofthese ions. Ions of As, V, Mo, W, occupy tetrahedral positions, however,with that, the octahedral positions are strongly distorted. According toZagumiennyi, the decrease in the light output is also a result of use asa source reagent of Lu₂O₃ with the purity of 99.9% (or less) instead ofreagent Lu₂O₃ with a purity of 99.99% (or higher). Some admixture withthe source reagent Lu₂O₃ with the 99.9%-or-less-pure Lu₂O₃ can decreasethe light output luminescence 2-10 times. The decrease in the lightoutput occurs due to the formation of Ce⁴⁺ ions in heterovalentsubstitution which takes place during the growth of crystal on thebackground of crystallization. Zagumiennyi further identified thesimplest schemes of substitutions that have either optimum or harmfuleffect on the crystal performance:

-   -   1) Lu³⁺+Si⁴⁺→Ce³⁺+S⁴⁺—optimal substitution of lutetium ions by        cerium ions,    -   2) Lu³⁺+Si⁴⁺→Ce⁴⁺+Me³⁺—highly probable, harmful and undesirable        heterovalent substitution with the compensation of charge for        admixture of Me³⁺≦Be, B, Al, Cr, Mn, Fe, Co, Ga, In.    -   3) 2Lu³⁺→Ce⁴⁺+Me²⁺—highly probable, harmful and undesirable        heterovalent substitution with the compensation of charge for        admixture Me²⁺=Mg, Ca, Mn, Co, Fe, Zn, Sr, Cd, Ba, Hg, Pb.    -   4) 3Lu³⁺→Ce⁴⁺+Ce⁴⁺+Me¹⁺—probable, harmful and undesirable        heterovalent substitution with the compensation of charge at big        concentration of cerium ions for admixture of Me⁺=Li, Na, K, Cu,        Rb, Cs, Tl.

Zagumiennyi also teaches that the additional introduction into the meltof at least one of the chemical compounds (for example, oxide) of theelements of the group Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W, in theamount 2-3 times greater than the total concentration of admixture ions(Me⁺+Me³⁺+Me³⁺) eliminates the formation of Ce⁴⁺ ions in the process ofthe crystal growth.

Zagumiennyi is focused on primary improvement in light output of thematerials with crystallographic structure of LSO by elimination of Ce⁴⁺ions and on control of Ce³⁺ concentration in LuO₇ polyhedrons. He alsoteaches that by increasing the concentration of Ce³⁺ ions in LuO₇polyhedrons it is possible to decrease decay time of scintillation pulsedown to 30 ns for LSO material. In accordance to Zagumiennyi's teachingthis effect can be achieved by codoping of LSO material with Zr, Sn, Hf,As, V, Nb, Sb, Ta, Mo and W.

On the other hand Spurrier at al. (Spurrier M, Melcher C. L.,Szupryczynski P., Carey A. A. “Lutetium oxyorthosilicate scintillatorhaving improved scintillation and optical properties and method ofmaking the same,” U.S. patent application Ser. No. 11/842,813 disclosesthe role of Ca²⁺ and other divalent metal ions such as Ba, Mg, Sr, andtheir positive effect on light output and decay time of LSO. Contrary toZagumiennyi's teaching which regards divalent metal ions “harmful andundesirable heterovalent substitutions” for Lu³⁺, Spurrier at al.(Spurrier M. A., Szupryczynski P., Yang K. Carey A. A., Melcher C. L.,Effects of Ca²⁺ codoping on the scintillation properties of LSO:Ce, IEEETrans. Nucl. Sci., vol 55 no. 3 (2008) 1178-1182) provides supportingexperimental data showing the significant increase in light output ofLSO:Ce,Ca (38,800 photons/MeV) compared to LSO:Ce (30,900 photons/MeV).Moreover, the increase in concentration of Ca²⁺ codopant in LSO:Ce,Caresults in shortened decay time (as short as 31 ns for Ca concentration0.3-0.4%). The Spurrier references show that increase in Ca²⁺concentration changes the relative concentrations of Ce³⁺ ions betweentwo cerium sites, referred to by Zagumennyi as “LuO₇ and LuO₆polyhedrons.” The Spurrier references teach that the presence of Ca²⁺ inthe structure of LSO also compensates for oxygen vacancies that wouldotherwise diminish light output by trapping electrons in competition toCe³⁺. This possibility is supported by the importance of anoxygen-containing growth atmosphere. A further possibility, inaccordance to the Spurrier references, is that a presence of Ca²⁺suppresses an as yet unidentified trapping center. However, unless Ca²⁺interacts preferentially with only one of the two cerium sites, suchinteractions would seem to primarily pertain to increase light outputrather than faster decay time. The Spurrier references disclose that thedecay time of LSO:Ce,Ca can be tuned between 31 ns up to 43 ns. However,higher concentrations of Ca²⁺ necessary to achieve short decay timesresult in significant problems with crystal growth stability causingsevere crystal deformation and cracking. These problems are related tothe changes in surface tension properties of the melt that results indifficulties in maintaining stable crystal-melt interface in theCzochralski growth process of LSO. Spurrier et al. (Spurrier M. A.,Szupryczynski P., Rothfuss H., Yang K., Carey A. A., Melcher C. L., “Theeffect of codoping on the growth stability and scintillation propertiesof LSO:Ce”, Journal of Crystal Growth 310, (2008) 2110-2114) proposes amethod for controlling of crystal-melt interface by codoping with Zn.While relatively high concentrations of Zn improve growth stability, thelow boiling point of Zn results in evaporation of Zn from the melt anddifficulties in controlling its concentration in the melt. The problemis even more severe during growth of large diameter commercial size LSOboules, where relatively large exposed area of the melt causes anincrease in evaporation of Zn.

To reduce or eliminate the shortcomings of the prior art, the presentdisclosure discloses a composition of oxyorthosilicate materials dopedwith cerium, and with additional codopants of various elements selectedfrom groups IIA and IIB periodic table of elements (Mg, Ca, Sr, Ba, Zn,Cd). The examples disclosed herein allow one to achieve a tunable decaytime of scintillation pulse within a broader range (between about 30 nsand about 49 ns) than reported to date (see, the Spurrier 2008 paper).Additionally, they allow a better control over the crystal productionprocess by reducing or eliminating growth instability problems andminimizing production losses due to crystal cracking that was reportedfor high concentration of codopants (see, the Spurrier 2008 paper).Moreover, they provide methods of improving light output and uniformityof codoped crystals with optimized high light output and scintillationtime profile that can be tuned to very specific scintillatorapplications. As a result of better control of the oxyorthosilicatematerial production process the overall crystal production cost can belowered that opens new avenue for further development of scintillatorcrystal based radiation detection technologies.

The general chemical formula for an example LSO material is

Ce_(2x)A_(2y)Lu_(2(1-x-y))SiO₅,

where A denotes one or more divalent element from the group: Mg, Ca, Sr,Ba, Zn, Cd or any combination thereof. Here, A is a codopant that isused in conjunction with an activator ion (preferably, as in thisexample, Ce) in specific concentration ratios to tune the scintillatordecay time and to achieve optimum other scintillation properties. Inthis example x is greater or equal to 0.00001 and less or equal to 0.1(i.e., from about 0.001% to about 10%), and y is greater or equal to0.00001 and less or equal to 0.1 (i.e., from about 0.001% to about 10%).

Preliminary experiments on full size production boules (80 mm or largerin diameter) show that codoping with Mg, Sr and Zn in low concentrations(i.e., concentration of codopant at least 10 times lower thanconcentration of Ce) results in an increase in the decay time of LSO(approaching ˜50 ns), as shown in Example 7, 8 and 9 below. Moreover,higher concentrations of Mg and Sr (i.e, concentration of codopants atleast 3 times higher than concentration of Ce) can result in a decaytime of as short as reported by Spurrier (Spurrier M, Melcher C. L.,Szupryczynski P., Carey A. A. Lutetium oxyorthosilicate scintillatorhaving improved scintillation and optical properties and method ofmaking the same” U.S. patent application Ser. No. 11/842,813) with Cacodopant, as shown in Examples 4 and 5 below. In contrast, lowerconcentrations of Ca (relative to Ce) does not result in longerscintillation decay time (longer than 41 ns, see example 6 below) asmentioned above with the other elements from groups IIA and IIB. Thepresent disclosure includes examples of codoping schemes that includesspecific combinations of selected codopants introduced to the melt intheir predetermined relative concentrations. These concentrations aredefined relative to the concentration of Ce³⁺ and other codopants.Additionally, the concentration of the Ce³⁺ is adjusted relatively tothe overall concentration of codopants in the melt from which crystal ispulled to minimize the surface tension effects. Controlling the codopantratios can optimize the performance of the resultant material, namely:optimum light output, fast rise time with short decay time (approaching˜30 ns), or optimum light output, fast rise time with long decay time(approaching ˜50 ns). Appropriate adjustments of the concentration ofCe³⁺ relative to the concentrations of IIA and/or IIB elements enablemaintaining the stability of the crystal growth process (see, Examples 1and 2 below). The following schemes are examples of concentrationscerium and other codopants from group IIA and IIB for achieving improvedscintillation characteristics of LSO:

-   -   Scheme 1. LSO with slow decay time (approaching ˜50 ns): cerium        concentration 0.1% or higher, with elements Mg, Sr, Zn or Cd        present less than 1/10 of the Ce concentration.    -   Scheme 2. LSO with fast decay time (approaching ˜30 ns): cerium        concentration 0.05% or less, and Ca, Mg, Sr, Zn, or Cd more than        3 times of the Ce concentration.    -   Scheme 3. LSO with exceptional uniformity and exceptional light        output: Cerium concentration 0.2% or higher, and additional Mg        and Ca, or Mg and Sr or Ca and Sr in concentrations less than ¼        for of the Ce concentration.

Moreover, the specific concentration codoping schemes above arebeneficial for controlling the scintillation time profile that isfavorable for TOF PET applications. In accordance to the Shao referenceabove, fast rise times, along with short decay times and high lightoutput are critical parameters for achieving the best timingcharacteristics of the TOF PET detectors. The best time resolution wasexperimentally achieved with detectors built from LSO crystals grownfollowing scheme 2, in good agreement with Shao. These crystals havefaster rise time, shorter decay time and relatively high light outputcompared to the other LSO compositions synthesized based on scheme 1 and3.

Additional benefits of having the ability to produce scintillators withtuned decay time is the ability of utilizing a DOI information (Depth ofInteraction) and pulse shape discrimination techniques in a “phoswich”configurations of a PET detector block. In a phoswich configuration, acrystal element includes two or more crystals with differentscintillation decay times. Maximum spread in the decay time of ˜20 nsallows one to clearly distinguish between different sections of thecrystal element and minimize a parallax effect in the imagereconstruction algorithms.

Experimental work done on other oxyorthosilicates produces similarbenefits to these described above. Example 11 shows the data obtainedfor LYSO (LSO with intentionally added Yttrium) crystal grown with Caconcentration higher than Ce produced a short decay time. Furtherincrease in Ca content in accordance to scheme 2 resulted in decrease indecay time similar to this observed in LSO without Yttrium.

Example 12 shows LYSO composition codoped with low concentrations of Caand Mg relatively to Ce. This composition results in an increase in thedecay time very similar to observed in LSO material.

The families of oxyorthosilicate scintillators proposed in this work canbe successfully applied in any gamma/x-ray detector system regardless ofthe type of the optical sensor chosen. Examples include detectors forthe field of Medical Diagnosis (PET, PET/CT, SPECT, SPECT/CT, MR/PETsystems), particularly in TOF PET systems, well logging industry (oilwell logging probes), and homeland security applications.

II. Example Configurations Examples 1-12

LSO scintillator crystals were grown using the well-known Czochralskiprocess (cited above). Starting materials Lu₂O₃, SiO₂, CeO₂, ZnO, CaO,MgO, SrCO₃, Y₂O₃ were at least 99.99% pure. Nominal concentrations ofcodopants in the melt were adjusted according to codoping schemes 1-3discussed above. The actual concentration of codopants in the crystalmay differ from the concentration in the melt due to the solid-liquidsegregation and the fraction of the melt solidified. The crystals weregrown with pull rate ˜3 mm per hour, with a rotation rate 1 rpm. Thegrowth atmosphere composition during crystal growth and cooldown wasmaintained constant with approximately one percent of oxygen in bulknitrogen. The crystals were grown to about 80 mm in diameter and about240 mm in length. The slabs had 20 mm in thickness and were cut andnumbered, starting from the bottom section of the crystal boule. Lightoutput measurements were done under excitation with Cs¹³⁷ gamma source(662 keV). The scintillation light was collected using Hamamatsu R877photomultiplier. The results are presented using arbitrary scale definedby numbers of channels of MCA (Multichannel Analyzer) unit used in themeasurements. BGO crystal was used as a reference (BGO photopeak wasmeasured at 100 channel position). Decay time was measured using zerocrossing method.

Example 1

FIG. 1 shows a LSO crystal boule with high concentrations of 0.1% Ce and0.05% Ca. Crystal growth instability problems occurred during growthprocess. This crystal was grown using the art of Spurrier (U.S. patentapplication Ser. No. 11/842,813) applied to a commercial size LSO boule.

Example 2

FIG. 2 shows a LSO crystal boule with Ce and Ca with appropriateadjustments of cerium and calcium concentrations according to scheme 2.The commercial size crystal boule was grown under stable growthconditions, with 0.025% Ce and 0.1% Ca, respectively.

In example 1 cerium concentration is relatively high, calcium content isa factor of 2 lower than cerium. However, a combined effect of Ce and Caintroduces instabilities in crystal-melt interface causing severecrystal cracking and loss of control over the crystal growth process. Inthe example 2, Ce concentration is 4 times lower, and calciumconcentration is 2 times higher than in the example 1. However, combinedeffect of Ce and Ca does not affect crystal growth stability. Higherconcentration of Ca over Ce is necessary to achieve scintillation timeprofile that is favorable for application in TOF PET.

Example 3

Table 1 shows the light output and decay time measured for LSO crystalwith an adjusted Ca to Ce concentrations ratio (Ce 0.033%, Ca 0.1%)according to scheme 2.

TABLE 1 Light Decay Slab Output* Time number [chMCA] [ns] 1 527 32.6 2501 32.6 3 492 32.7 4 500 32.6 5 505 33.8 6 503 32.6 7 512 34 8 505 32.59 548 33.9 10 515 32.6 11 517 34 12 527 32.6 13 519 32.5 *Light outputwas measured relative to BGO crystal reference light output, with aphotopeak position at 100 channel MCA.

Example 4

Table 2 shows the light output and decay time measured for LSO crystalwith adjusted Mg to Ce concentrations ratio (Ce 0.025%, Mg 0.1%)according to scheme 2.

TABLE 2 Light Decay Slab Output* Time number [chMCA] [ns] 1 601 34.8 2633 34.6 3 625 35.4 4 635 35 5 605 36 6 668 35.7 7 609 35.9 8 646 35.8 9620 35.8 10 632 36

Example 5

Table 3 shows the light output and decay time measured for LSO with highconcentration of strontium (Ce 0.025%, Sr 0.1%) according to scheme 2.

TABLE 3 Light Decay Slab Output* Time number [chMCA] [ns] 1 653 37 2 58334.3 3 653 36.6 4 586 34.6 5 627 36.4 6 576 34.8 7 604 36.4 8 576 36.2 9625 36.1 10 605 36.4 11 581 34.6 12 581 36.4

Example 6

Table 4 shows the light output and decay time measured for an LSO withthe lower concentration of Ca (Ce 0.1%, Ca 0.05%).

TABLE 4 Light Decay Slab Output* Time number [chMCA] [ns] 1 451 36.75 2454 37.59 3 491 37.4 4 511 37.56 5 536 38.01 6 560 38.28 7 554 37.84 8559 38.65 9 567 38.83 10 570 39.08 11 574 39.4 12 596 38.98

Example 7

Table 5 shows the light output and decay time measured for an LSO with alow concentration of Mg (Ce 0.35%, Mg 0.01%) according to scheme 1.

TABLE 5 Light Decay Slab Output* Time number [chMCA] [ns] 1 628 48.1 2633 47.9 3 603 47.7 4 644 47.7 5 609 47.4 6 650 48 7 615 47.8 8 612 47 9593 46.1 10 644 47.1 11 630 45.9 12 634 47.6

Example 8

Table 6 shows the light output and decay time measured for an

LSO with a low concentration of Sr (Ce 0.2%, Sr 0.02%) according toscheme 1.

TABLE 6 Light Decay Slab Output* Time number [chMCA] [ns] 1 507 47.7 2511 47.6 3 518 47.7 4 556 48.6 5 568 48.3 6 558 47.8 7 552 47.8 8 55947.9 9 566 47.9 10 563 47.6 11 572 47.1 12 517 47.7

Example 9

Table 7 shows the light output and decay time measured for an LSOcodoped with Ce 0.2% and low concentrations of Zn 0.05%. Theconcentration of Zn is calculated at temperatures below vaporizationpoint of Zn compound used in the experiment). Since the vaporizationrate of Zn at temperatures exceeding 2000° C. (approaching melting pointof LSO) is very high, the effective concentration of Zn in the meltduring crystal growth process drops significantly. The composition ofthe melt during crystal growth process that reflects the concentrationranges described by scheme 1.

TABLE 7 Light Decay Slab Output* Time number [chMCA] [ns] 1 663 45.9 2593 46.7 3 606 47.1 4 617 46.8 5 599 46.8 6 597 46.9 7 568 46.5 8 59946.7 9 596 46.6 10 587 46.5 11 590 46.6

Example 10

Table 8 shows the light output and decay time measured for an LSO with alow concentration of Mg and low concentration of Ca (Ce 0.2%, Mg 0.015%,Ca 0.01%) according to scheme 3.

TABLE 8 Light Decay Slab Output* Time number [chMCA] [ns] 1 605 47.8 2584 47.5 3 562 48.1 4 569 47.9 5 575 47.7 6 590 47.1 7 568 47.4 8 58447.8 9 602 47.7 10 613 47.8 11 598 47.9 12 589 47.7 13 579 47.9

Example 11

Table 9 shows the light output, decay time and energy resolutionmeasured for an LYSO with relatively high concentration of Ca (Ce 0.1%,Ca 0.15%, Y 5%).

TABLE 9 Light Decay Slab Output* Time number [chMCA] [ns] 1 359 35.8 2405 36 3 409 35.9 4 402 36.3 5 463 36.8 6 501 36.9 7 485 36.7

Example 12

Table 10 shows the light output and decay time measured for an LYSOdoped with Ce 0.2%, Ca 0.01%, Mg 0.015% and Y 1%. Variations in decaytime between slab 1 through slab 10, we believe are due to differencesin segregation coefficients between Mg and Ca while the Mg is depletedand melt becomes increasingly enriched in Ca.

TABLE 10 Light Decay Slab Output* Time number [chMCA] [ns] 1 369 43.1 2434 43.1 3 520 45.4 4 547 45.7 5 564 46 6 546 45.8 7 579 47 8 574 47.5 9570 47.1 10 556 45.7

III. Summary

Thus, LSO scintillator crystals with high light yield have been producedaccording to the present disclosure. Codoping of cerium and one or moreof ions from Groups IIA and/or IIB can be made in predetermined ratiosof concentration to achieve desired properties of the crystal andcrystal growth stability.

Because many embodiments of the invention can be made without departingfrom the spirit and scope of the invention, the invention resides in theclaims hereinafter appended.

1. A method of growing a single-crystalline scintillator material from amelt having a composition of the formula,Ln_(2x)A_(2y)Lu_(2(1-x-y))SiO₅, wherein Ln consists essentially of oneor more lanthanides, one or more actinides or a combination thereof, Aconsists essentially of one or more Group-IIA or -IIB elements of theperiodic table of elements or any combination thereof, the methodcomprising: selecting a fluorescence decay time between about 30 ns andabout 50 ns, inclusive, to be achieved for the grown single-crystallinematerial; based on the decay time to be achieved, determining a relativevalue between x and y, wherein x is greater than or equal to 0.00001 andless than or equal to 0.1, and y is greater than or equal to 0.00001 andless than or equal to 0.1 so as to achieve stable growth of thesingle-crystalline scintillator material from the melt; and growing asingle-crystalline scintillator material from the melt with the relativevalue between x and y.
 2. The method of claim 1, wherein: Ln consistsessentially of Ce, Pr, Th, Eu, Tb or any combination thereof; and Aconsists essentially of Be, Mg, Ca, Sr, Ba, Zn, Cd or any combinationthereof.
 3. The method of claim 2, wherein Ln consists essentially ofCe.
 4. The method of claim 3, wherein A consists essentially of Mg, Ca,Sr or any combination thereof.
 5. The method of claim 3, wherein: x isgreater than or equal to 0.001, and A consists essentially of Mg, Sr, Znor Cd or any combination thereof, wherein y is about 1/10 of x orsmaller.
 6. The method of claim 3, wherein: x is greater than or equalto 0.002, A consists essentially of a combination of Mg and Ca, or of Mgand Sr, or of Ca and Sr wherein y is about ¼ of x or less.
 7. Asingle-crystalline scintillator material grown from a melt having acomposition of the formula, Ln_(2x)A_(2y)Lu_(2(1-x-y))SiO₅, wherein: Lnconsists essentially of one or more lanthanides, one or more actinidesor a combination thereof, A consists essentially of one or moreGroup-IIA or -IIB elements of the periodic table of elements or anycombination thereof, x is greater than or equal to 0.00001 and less thanor equal to 0.002, and A consists essentially of Ca, Mg, Sr, Zn or Cd orany combination thereof, y is greater than or equal to 0.00001 and lessthan or equal to 0.1 and is about three times x or greater.
 8. Asingle-crystalline scintillator material grown from a melt having acomposition of the formula, Ln_(2x)A_(2y)Lu_(2(1-x-y))SiO₅, wherein: Lnconsists essentially of one or more lanthanides, one or more actinidesor a combination thereof, A consists essentially of one or moreGroup-IIA or -IIB elements of the periodic table of elements or anycombination thereof, x is greater than or equal to 0.002 and less thanor equal to 0.1, and A consists essentially of a combination of Mg andCa, Mg and Sr, or Ca and Sr, wherein y is about ¼ of x or less.
 9. Thesingle-crystalline scintillator material of claim 8, wherein: Lnconsists essentially of Ce, and A consists essentially of Mg.
 10. Themethod of claim 5, wherein x is greater than or equal to about 0.004.11. The method of claim 6, wherein x is greater than or equal to about0.008.
 12. The single-crystalline scintillator material of claim 8,wherein x is greater than or equal to about 0.008.
 13. Thesingle-crystalline scintillator material of claim 9, wherein x isgreater than or equal to about 0.008.
 14. A single-crystallinescintillator material made by the method of claim 1, wherein: x isgreater than or equal to about 0.004 and less than or equal to 0.1, andy is about 1/10 of x or less.
 15. The single-crystalline scintillatormaterial of claim 14, wherein: Ln consists essentially of Ce, and Aconsists essentially of Mg, Sr, Zn, Cd or a combination thereof.